1. Field of the Invention
The present invention relates to an auto focus unit and a camera that adjusts the position of a focus lens in an optical system automatically so that an optical image of an object appearing through the optical system can be brought into focus.
2. Description of the Related Art
A recent digital camera has an auto focus function. Several different methods exist for carrying out this auto focus function, one of which is briefly explained below. A scanning area is fixed upon the light-receiving surface of an imaging device where optical images are captured. Whenever the imaging device captures the optical image, a focus lens is moved along an optical axis and a contrast value in the scanning area is detected. The detected contrast values corresponding to a plurality of positions of the focus lens are recorded in a memory. One position of the focus lens, of which the contrast value is highest among the recorded contrast values, is selected as the in-focus position of the focus lens, and the focus lens is moved to that position. In the above method, it is necessary to consider a plurality of detected contrast values in the fixed scanning area.
In addition, an image-pursuit function is known. By the image-pursuit function, the image of a targeted object that moves in a space corresponding to an entire image to be captured is pursued from a plurality of sequentially captured image. A moving object within the entire image to be captured can be focused on by carrying out the auto focus function in cooperation with the image-pursuit function. For example, the scanning area is moved to capture an optical image of a targeted object according to the movements of the targeted object within the entire image by carrying out the image-pursuit function, while the auto focus function is carried out based on the moved scanning area. In order to auto focus on a moving object, a plurality of optical images should be captured so that a plurality of contrast values can be detected before the scanning area is moved.
However, the scanning area may be moved before the detection of a plurality of contrast values if the quantity of a hand shake or magnification of a photographic optical system is large. Consequently, it is problematic to auto focus on a moving an object in an unstable manner.